A trademark can be a sign, a design, or an expression that identifies products from a particular manufacturer. It can also be used as a verb, to indicate that a company or individual has registered a trademark, ensuring that no one else may use it without permission.
Many products feature a printed warning, letting consumers know that they cannot reproduce a product's trademark in any way. Coca Cola is one company that very actively pursues trademark violations.
Once Millie realized that the t-shirts she was selling under the name "Shirts Aplenty" were selling well locally, she decided to trademark that name and her logo, which was a shirt with eight arms.
Metaphorically, a trademark is a distinctive mark, appearance, or action that is associated with a particular person or group. You can use trademark in this metaphorical way as an adjective as well. Something that is a perfect example of whatever it is might be called a trademark example of that thing.
When my father saw that the lettuce plants in his garden had all been very neatly nibbled down to the stem, he said that this was a trademark example of the damage rabbits can do to vegetables.